The present invention generally relates to the conversion of chemical energy to electrical energy. More particularly, this invention relates to a method for estimating the discharge curve of an electrochemical cell having a lithium anode and a solid cathode. A preferred couple is a lithium/silver vanadium oxide (Li/SVO) cell. In such cells, it is desirable to accurately estimate the shape of the discharge curve, especially in the cell's latter stages of life, from the discharge curves of other similarly constructed cells subjected to varied accelerated discharge protocols.
In implantable medical device applications, it is important to accurately predict when the power source will reach its end of life. For patient safety, doctors need to have this information several months before the power source reaches the end of its service life. This gives them time to schedule the patient for replacement of the implantable device. In that respect, the Li/SVO cell system is typically used as the power source for implantable cardiac defibrillator applications requiring high rate pulse capability, i.e., about 1 to about 5 amps. Due to the characteristic voltage delay and Rdc growth that occurs at about the 2.6-volt plateau, a pre-determined Rdc or voltage value under high current pulsing is sometimes used as an elective replacement indicator (ERI). This pre-determined voltage value varies depending on the cell size, theoretical capacity and the associated device design. Consequently, selection of the ERI reading is very complicated and dependent on the individual device design of each manufacturer.
For that reason, the present method enables the designer of an implantable medical device to accurately predict the shape of a cell's discharge curve, especially during latter stages of its life, from similarly constructed cells discharged under relatively rapid discharge regimes. This helps the designer know when a power cell model is approaching its end of life under varied usages. The device electronics can then be programmed to trigger when a pre-determined Rdc or current pulse voltage has been reached as the elective replacement indicator.